Ratioed measurements are measurements of one received signal relative to another. For example: an 'A/R1' measurement trace's data is computed as the A receiver's measured complex data divided by the R1 receiver's measured complex data. S-parameters by their definition are also ratioed measurements. In that Application Note, what the author meant by "single receiver channel" is just a measurement of one receiver. For example: 'A' or 'R1'. The Averaging topic in the PNA online Help also makes essentially the same statement you quoted but goes into a little more detail (look for the heading that begins with 'Unratioed').

If the author of the app note originally cautioned the reader about the PHASE format and averaging, his endeavour to show the reader how to make correct measurements would have been successful. I don't know why the author didn't just come out and explicitly say what was on his mind.

By the way, can you give me an example - an actual application - where someone would make a "single channel receiver" measurement. Can you in your reply assert in your example whether you are in S11 or S21 mode? Thanks.

By the way, can you give me an example - an actual application - where someone would make a "single channel receiver" measurement. Can you in your reply assert in your example whether you are in S11 or S21 mode? "

Single channel receiver measurements are typically performed to measure power (either incident to the DUT, reference receivers, or delivered from the DUT, measurement receivers). these measurements are typically calibrated with a receiver response cal, which means at some point you would have done a source power cal with a power meter and the response cal transfers the calibration with the power meter to the VNA receivers, turning the VNA receiver to a very fast, accurate, and tunable power meter.